docs: def _extract_local_mean_gauss(image, mask = slice(None), sigma = 1, voxelspacing = None):
      """
      Internal, single-image version of `local_mean_gauss`.
      """
      # set voxel spacing
      if voxelspacing is None:
          voxelspacing = [1.] * image.ndim
          
      # determine gaussian kernel size in voxel units
      sigma = _create_structure_array(sigma, voxelspacing)
          
      return _extract_intensities(gaussian_filter(image, sigma), mask)